Unexpected Performance of a Bifunctional Sensitizer/Activator Component for Photon Energy Management via Upconversion

We here report on the observation of upconverted photoluminescence (UC-PL) from the blue-light-emitting 9,10-diphenylanthracene (DPA) mixed with the yellow-light-absorbing bifunctional sensitizer/activator component of (3,3,7,8,12,13,17,18-octaethylporphyrin-22,24-diid-2-one) PtII (PtOEP-K). Yellow-to-blue UC-PL (0.680 eV spectral upshift) is achieved at room temperature under ultralow power continuous incoherent photoexcitation (220 μW/cm2) despite the absence of triplet energy transfer (TET) between PtOEP-K and DPA. Under selective CW-laser photoexcitation of PtOEP-K in DPA:PtOEP-K, a 2.5% UC-PL quantum yield is obtained; that is an improvement exceeding by more than 3 orders of magnitude the UC-PL quantum yield of TTA-UC material combinations wherein no TET is operative. The PL response of DPA:PtOEP-K to varying laser fluence suggests that bimolecular annihilation reactions between triplet-excited PtOEP-K facilitate the UC-PL activation in DPA. These findings pave the way toward low-complexity strategies for the reduction of transmission losses in solar energy technologies through an innovative wavelength upshifting protocol involving excitonic materials.


Time-integrated PL characterization under incoherent photoexcitation:
A Hg/Xe light source (Research F/2.2 Illumination Source 200 W Hg/Xe Ozone-free Arc Lamp, Newport) was used for photoexciting at 60040 nm.Care was taken to ensure the elimination of any parasitic photoexcitation of the DPA component by high photon energy residues of the lamp output, by rejecting the high photon energy portion of the Hg-Xe lamp output with a combination of a band pass (FB600-40, Thorlabs) and a long pass (FG550S, Thorlabs) filter.The photoexcitation intensity used was determined based on a calibrating photodiode (818-UV/DB UV Detector, Newport DB15 Calibration Module) with known responsivity.The generated luminescence was detected in the 45040 nm spectral region through a band pass filter (FB450-40, Thorlabs).The PL spectra were collected with a Newton CCD camera (DU920P-BEX2, Oxford Instruments) coupled to a Czerny-Turner spectrograph (Kymera-328I-B1-SIL, 328 mm focal length, F/4.1 aperture, Oxford Instruments).
PL quantum yield (PLQY) determination: PLQY data were obtained by using a Rhodamine 6G (R6G) dilute solution in ethanol as a reference, after photoexcitation under 532 nm.The molarity of the R6G solution was kept low so that a measurable optical density lower than 0.05 was obtained at the photoexcitation wavelength.The UV-vis absorption and PL data of the reference solution were identically recorded as the DPA:PtOEP-K solutions in toluene.PLQY values were calculated via the following equation, whereby Q, I, OD and n represent quantum yield, PL intensity, optical density and solvent refractive index, respectively.
Indicator R corresponds to the reference solution for which a QR of 94% was considered. of PtOEP-K without imposing any symmetry constraints.Subsequently, singlet (S0 → Sn) and triplet (S0 → Tn) vertical excitation energies were computed from linear response time-dependent density functional theory (TD-DFT).Solvent effects on the electronic spectra are introduced using the polarisable continuum model with toluene as a solvent.The lowest energy singlet (S1 → S0) and triplet (T1 → S0) emission energies were computed by optimizing the corresponding S1 and T1 excited state geometries.Quantum chemical calculations were performed with the Gaussian 16 software.[1] S-5

Transition
Energy      DPA:PtOEP (30 mM:150 μM) solutions in degassed toluene after photoexcitation at 532 nm (green line) and 590 nm (orange line).In all cases photoexcitation was with a 10 Hz train of pulses of a Nd:YAG/OPO laser source (6-8 ns laser pulse).All PL spectra were collected with a Newton CCD camera (DU920P-BEX2) coupled to a Czerny-Turner spectrograph (Kymera-328I-B1-SIL, 328 mm focal length, F/4.1 aperture) with an exposure time of 0.25 s after 10 accumulations.Dedicated notch filters were used to suppress the PL intensity of the laser excitation lines; a NF01-532U (Shemrock) for λexc.=532 nm and a ZET594TopNotch (Chroma) for λexc.=590 nm).The averaged laser pulse energy of photoexcitation was determined with a J-10MB-HE pyroelectric sensor coupled to a FieldMax II-TOP power/energy meter (Coherent Inc.) Density functional theory (DFT) calculations: DFT was used with the dual-range local exchangecorrelation functional M11-L and a mixed basis set, the triple-ζ Pople 6-311++G(d,p) basis set for C, H, N and O atoms and a Lanl2DZ basis with effective core potential for Pt to obtain the ground state geometry S-4

Figure S4 .
Figure S4.Downfield expansion of the 1 H NMR spectra presented in Figure S1.

Figure S8 .
Figure S8.Room temperature, time-integrated PL spectra of a) DPA:PtOEP-K (30 mM:150 μM) and b) DPA:PtOEP (30 mM:150 μM) solutions in degassed toluene after photoexcitation at 532 nm (green line) and 590 nm (orange line).In all cases photoexcitation was with a 10 Hz train of pulses of a Nd:YAG/OPO laser source (6-8 ns laser pulse).All PL spectra were collected with a Newton CCD camera (DU920P-BEX2) coupled to a Czerny-Turner spectrograph (Kymera-328I-B1-SIL, 328 mm focal length, F/4.1 aperture) with an exposure time of 0.25 s after 10 accumulations.Dedicated notch filters were used to suppress the PL intensity of the laser excitation lines; a NF01-532U (Shemrock) for λexc.=532 nm and a ZET594TopNotch (Chroma) for λexc.=590 nm).The averaged laser pulse energy of photoexcitation was determined with a J-10MB-HE pyroelectric sensor coupled to a FieldMax II-TOP power/energy meter (Coherent Inc.)

Figure
FigureS9presents a TTA-UC system comprising an Emitter (E) mixed with a bifunctional

Figure S9 .
Figure S9.Jablonksi diagram depicting the main photophysical processes that lead to TTA-UC PL activation through a bifunctional sensitizer/activator component.
constant of the Sensitizer component, respectively.Monomolecular relaxation takes place via both radiative and non-radiative deactivation of the T1 state, therefore    = (   +    ) where    and    correspond to the radiative and non-radiative rate constant of the Sensitizer, respectively.Triplet-triplet annihilation reactions in the Sensitizer component result in the activation of the higher-lying energy state D * with energy   * ≤ 2 ×   1 from where energy can be transferred to the first singlet excited state S1 of the Emitter component with a rate constant   .The rate law dictating the activation and deactivation of the D * state is described by Equation 3. ]  corresponds to the population of Sensitizer component in the D * state.Considering a concentration of Emitter species available to accept the energy from the D * state of the sensitizer, the apparent monomolecular deactivation rate constant   of D * corresponds to   =   +    1 [ 0 ]  .Here,   is the internal conversion deactivation rate of the D * state and  1 is the fraction of the ground state S0 Emitter molecules adjacent to the D * -excited Sensitizer species with 0 ≤  1 ≤ 1.The rate law dictating the activation and deactivation of the S1 state of the Emitter species is described by Equation 4.    2 [ 0 ]  ], where    and    correspond to the radiative and non-radiative rate constant of the Sensitizer, respectively.The fraction  2 [ 0 ]  of the Sensitizer molecules adjacent to the S1-excited Emitter species, where 0 ≤  2 ≤ 1, can serve as an energy sink during the resonance back energy transfer process from the S1 state of the Emitter with a back-energy transfer rate constant   .The fluence dependence of the sensitizer phosphorescence and the emitter TTA-UC luminescence is described by two major regimes.i) The apparent monomolecular rate relaxation constant dominates over the bimolecular kTTA[ 1 ]  term; that is    >> kTTA[ 1 ]  in Equation 5; the Sensitizer phosphorescence scales linearly with fluence:  1 ]  term overrides apparent monomolecular rate relaxation constant; that is    << kTTA[ 1 ]  in Equation 5; the Sensitizer phosphorescence exhibits a square root dependence on fluence according to Equation 11: Eq. 11 and the Emitter TTA-UC luminescence scales linearly with fluence according to Equation 12. *